Sadfella, my heart goes out to you. How painful to be CO (cut off) by your son and not even get to meet your new GS or find out his name! I can barely imagine how much that must hurt.
I think it was brave - and wise - of you to come here and tell us about your troubles. I understand if you don't wish to tell us the details of the "falling out," but I hope, eventually, you find it in you to do so. No identifying details, of course, but just enough so we know what it is you two argued about.
You say you and your son have never argued - but you also say he has "changed" a lot since meeting his DW. Have their been tensions between you over these changes? Some disagreements over how they are doing things, even if they didn't end up in full-blown arguments until now? Maybe baby-related, about the child's name or their childbirth choices or arrangements, etc? (If it was about the baby, that would explain why he told your XW not to give you current info.) It's hard to get my mind around the idea that one argument would lead to an estrangement just like that, unless there had been a build-up of issues over time. But I know I could be wrong.
Perhaps you should just back off for now, as some posters have suggested, and wait until he reaches out to you again. Hopefully, he will since he clearly still loves you.
But if you feel the need to try one more time, then I agree w/ Namsnanny's suggestion. I would just add an apology for the thing(s) that upset him. You haven't said if you know what it was, but there must be some clues in that argument you had. If that doesn't get some kind of response, though, then, IMO, you will really have to step back and be patient.
Of course, XW is afraid that the same thing could happen to her if she gives you info about son and his family. He told her not to and it is HIS life and HIS family. She sees what happened to you and, quite rightly, I think, fears the same could happen to her if she goes against his wishes regarding his own family unit. IDKY some posters seem surprised at this. I sense you understand and won't pressure XW for info about your new GS, etc., even though I know you must be aching for some (I would be, too, if I were in your shoes).
I hope you have other things in your life to enjoy, meanwhile - your job, a hobby, a circle of friends, other relatives, etc. And please keep reaching out to us.